1. Field of the Invention
This invention relates in general to the field of optics, and more particularly to Fresnel lenses.
2. Description of the Prior Art
In 1748, Count Buffon proposed to grind out of a solid piece of glass a lens in steps or concentric zones, in order to reduce the thickness of the lens to a minimum. In 1822, Augustin Fresnel, for whom the Fresnel lens is named, constructed a lens in which the centers of curvature of the different rings receded from the axis according to their distances from the center, so as to practically eliminate spherical aberration.
Modern Fresnel lenses are used as light condensers, illuminators, and magnifiers, and in many other ways. Fresnel lenses basically consist of a series of concentric prismatic grooves, designed to cooperatively direct incident light rays to a common focus. This type of lens is thin, lightweight, and of high aperture. Also, this type of lens can be accurately mass-produced using replication techniques.
Generally, in prior art thin Fresnel lenses intended for visible light applications, the grooves are all the same width, so that the groove density is constant across the lens. The depth of the grooves increases as the distance between the groove and the center of the lens increases. The depth of the deepest groove places a limit on the minimum thickness of the lens. Therefore, if the depth of the grooves can be reduced, the thickness of the lens can be reduced.
One method of reducing groove depth is to increase groove density i.e., the number of grooves between the center of the lens and a given radius. However, diffraction effects limit the improvements which can be made in this way. Diffraction effects are especially strong in Fresnel lenses for infrared wavelengths, because the wavelength approaches the groove spacing.
Fresnel lenses are particularly useful with infrared energy rays. Such infrared rays are used in passive infrared burglar alarms, noncontacting thermometers, proximity detectors, and other devices. Lenses for collecting such infrared rays can be made of rather perfect, but expensive materials, such as germanium, but a desire to reduce costs often dictates cheaper, lossy materials, such as plastics. Lenses made from such materials must be very thin, or too much of the incident energy will be attenuated in passing through the lens. The shape of a conventional positive focal length lens is such that the center, the most ideal portion optically, is also the thickest portion. The Fresnel lens concept allows lenses to be made very thin, and adequately uniform in thickness from center to edge.